Hydraulic systems are utilized in many forms of construction equipment such as hydraulic excavators, backhoe loaders, and wheel loaders. The equipment is usually mobile having either wheels or tracks and includes a number of hydraulically actuated devices such as hydraulic cylinders and motors. In most cases, hydraulic systems are controlled by a valve arrangement in which a hydraulic pump provides pressurized fluid to a plurality of valves each associated with a hydraulic cylinder or motor. As an operator manipulates control levers located in the operator's compartment, hydraulic valves are controllably opened and closed such that pressurized fluid is controllably directed to the desired cylinder or motor.
When the rod/head assembly in a hydraulic cylinder is required to move in response to a operator command, it is important that it moves to the desired position in an accurate and robust manner. Achieving such accurate control is challenging because the system is fundamentally non-linear and is exposed to many disturbances including, inter alia, temperature changes, component wear, and varying external loads.
The most effective method of controlling hydraulic actuator systems is to use linear control theory. However, it is necessary to first linearize the system before linear control theory can be applied. Currently, the most common method of linearizing a system involves Taylor Series linearization whereby a system is linearized with respect to small perturbations in the states, inputs, and disturbances about a selected operating or equilibrium point. A linear control law can then be designed to provide good performance under the small perturbation constraint. This method's drawback is that predictable performance is only assured if the system stays close to the particular point about which it was linearized. It is generally accepted that controlling a nonlinear system with a linear control law based on a linear system that is constrained to an equilibrium point is undesirable for most hydraulic systems.
Gain scheduling is also presently used in the art to control a hydraulic actuator. The technique models the non-linear system as a plurality of linear systems centered about their selected operating or equilibrium points. Each linear system has an associated linear control law. In operation, when the system moves from one equilibrium point to another, the neighboring linear control laws are blended together. This approach is inherently discrete since a finite number of linear control laws are used to control a continuous motion of a nonlinear system. In addition, the software implementation of gain scheduling dramatically increases in complexity as the number of states and points of linearization increase.
The present invention is directed to overcoming one or more of the foregoing problems associated with known hydraulic control systems for cylinders.